1. Field of the Invention
The invention relates generally to printing apparatus and methods, and is specifically concerned with a printing apparatus employing an articulated mechanical arm, such as a robot arm. The invention also relates to a multicolor printing method which may be carried out using such apparatus.
2. Description of the Prior Art
In a typical product marking installation, the printing or marking device is situated at a fixed position with respect to the product conveyor and is arranged to print at a selected location on the surface of the product as the latter is brought into position by the conveyor. Various types of marking devices have been used for this purpose, including rotary devices and reciprocating or stamping mechanisms. Rotary printers are generally used when it is desired to print on moving objects, while reciprocating or stamping mechanisms are usually employed when the object is to be printed while it is temporarily at rest, as would occur on an intermittently moving conveyor.
With conventional types of product marking devices, it is generally necessary that the products be of uniform size and that they be oriented in the same way on the conveyor. Since the marking device is essentially dedicated to a particular type of product, a change from one product line to another usually requires that some mechanical alteration be made to the marking device or its mounting apparatus, in order to adapt to the different size or orientation of the new product. This presents difficulties in manufacturing plants where product lines are changed frequently, or in plants which have a number of different product lines.
Another difficulty with conventional product marking systems is that it is usually possible to print on only one face of the product, unless more than one marking device is employed. Moreover, the printed legend can usually be applied only to the most easily accessible surfaces of the product, which are usually the top and bottom surfaces of the product and the two side surfaces which lie parallel to the edges of the conveyor. The forward and rear faces of the product, which extend transverse to the conveyor direction, ordinarily cannot be printed because to do so would require mounting the marking device in a position at which it would obstruct the product movement on the conveyor.
A particularly useful type of printing for some types of product marking applications is pad transfer printing. This technique involves the use of an engraved printing plate and a resilient transfer pad, the latter usually consisting of silicone rubber. The printing plate is inked and then scraped so that the ink remains only in the engraved areas which define the legend to be printed. The resilient pad is brought first into contact with the printing plate, which releases the wet ink image to the pad, and is then brought into contact with the surface to be printed. The surface characteristics of the silicone rubber are such that the ink readily releases from the pad and adheres to the print receiving surface.
Pad transfer printing is a useful technique for printing precise or detailed legends, because the process does not rely on mechanical impact or friction and the image can therefore be transferred with very little distortion. In addition, the elastic deformation of the transfer pad allows an image to be transferred from a flat printing plate to virtually any type of raised or irregularly shaped surface, as well as to flat surfaces. The process is also useful for "wet on wet" printing of multicolor images, since the ink-repellent transfer pad will not disturb a previously deposited ink image when the next color is being printed on the substrate.
Despite the inherent advantages of pad transfer printing, existing types of pad transfer printing machines are not well suited to certain product marking applications. Typically, the transfer pad is carried by a mechanical linkage which moves the pad along a fixed path between the printing plate and a work station. With the exception of minor adjustments for pad height, compression distance, and the like, the path of movement of the pad is preset and cannot be altered to suit particular product marking applications. Thus, for example, if the printing stroke of the pad occurs in a vertically downward direction, as is usually the case, the printing apparatus must be mounted above the product conveyor and the products must be positioned with their print receiving surfaces facing upwardly. This may be difficult or impossible in certain situations. Pad transfer machines with swiveling pad mechanisms have been developed to allow printing on vertical or inclined surfaces, but these machines still allow only a limited range of pad movement and cannot be universally employed with any desired product shape or orientation.
When it is desired to print multicolor images using the pad transfer technique, it is usually necessary to employ a separate pad or printing machine for each color. This results from the fact that most pad transfer machines are arranged to print from only a single printing plate. When printing multicolor images on products carried by a moving product conveyor, the product must move from one printing station to the next in order to receive each successive color separation image. This can give rise to registration errors unless the movement of the conveyor is controlled very precisely.